It is shown that germanium is more rapidly etched than silicon in conventional fluorine-, chlorine-, and bromine-based low-pressure plasmas and that a high Ge/Si etch rate ratio ERR can be readily obtained. The etching of Ge induced by plasma-generated fluorine atoms relies much less on ion bombardment than the etching of Si. A very high Ge/Si ERR (=40) may therefore be achieved for plasma etching conditions or for reactive ion etching at high pressure (250 mtorr) where the importance of ion bombardment effects on the etching is reduced. For reactive ion etching at low pressure where the etching is strongly ion-enhanced, the Ge/Si etch rate ratio is reduced to =3. Silicon may be more rapidly etched than Ge in a gas mixture of SF6/JH2/CF4, A Si/Ge etch rate ratio greater than 70 is demonstrated in this work. Both the formation of carbonaceous material and an involatile germanium-sulfide compound on the Ge surface are important in reducing the reaction of fluorine with the substrate and lowering the Ge etch rate. It is also shown that this RIE approach may be used to selectively etch Si over a Si50Ge50 alloy. © 1991, The Electrochemical Society, Inc. All rights reserved.